The present invention relates generally to medical electrodes and, more particularly, to disposable medical electrodes intended for high-energy stimulation (i.e., defibrillation, pacing, and the like) with energy dispersion characteristics.
Medical electrodes provide an electrical interface between a patient and monitoring equipment (e.g., an electrocardiograph device) or between a patient and stimulating equipment (e.g., interferential and iontophoresis devices). A specific type of stimulating electrode, used to provide an electrical interface between a patient and defibrillation equipment, must be capable of conducting the high-energy level required for defibrillation. The present invention focuses on high-energy defibrillation and pacing electrodes. The general characteristics of, and distinctions among, monitoring electrodes, general stimulating electrodes, and defibrillation electrodes are outlined below.
A. Monitoring Electrodes
Medical monitoring electrode systems help to obtain desired physiologic responses for the assessment or treatment of diseases and injuries in humans. Monitoring electrodes are used to sense electrical signals, which are then transmitted to electrocardiograph (EKG), electroencephalograph (EEG), and electromyograph (EMG) devices. In general, monitoring electrodes for EKG, EEG, and EMG devices are small, for example on the order of a few square centimeters, because a relatively small contact area with a skin surface is sufficient for reception of electrical signals. Monitoring electrodes need only carry very low electrical signals: on the order of milliamps. In general, monitoring electrodes are not capable of conducting and distributing the high levels of energy required in transcutaneous stimulation and defibrillation electrodes.
Various x-ray transmissive monitoring electrodes have been made to facilitate x-ray examination of a patient without requiring removal of the electrodes or significantly impairing the x-ray image. For example, U.S. Pat. No. 5,265,579 issued to Ferrari discloses an x-ray transparent monitoring electrode and method for making that electrode. The electrode is used for continuous EKG monitoring. See column 1, line 55; column 2, line 8. A thin coating (a few microns in thickness; note that 1 mil=0.001 inches=0.0254 mm=25.4 microns) of silverxe2x80x94silver chloride 17a, 18a is applied, by silk screening, to a sheet of conductive carbon or graphite-filled polymer film that forms the x-ray translucent electrodes 17, 18. See column 3, line 67 to column 4, line 22. The x-ray translucent leads 24 have a tow of carbon fibers whose stripped ends are attached to the electrode using pressure-sensitive tape. See column 5, lines 51-54. The stripped ends are sandwiched between two portions of the tape. See column 7, lines 20-22. FIG. 2 of the patent illustrates that the stripped ends are fanned for attachment. A metallic coating such as nickel is applied to the carbon fibers. See column 6, lines 1-2.
B. Stimulating Electrodes
Stimulating electrodes emit electrical pulses for transcutaneous electrical devices, such as transcutaneous electrical nerve stimulation (TENS), electrical muscle stimulation (EMS), neuromuscular stimulation (NMS), functional electrical stimulation (FES), as well as interferential and iontophoresis therapy. Like monitoring electrodes, medical stimulating electrodes are also used to treat diseases and injuries in humans. Unlike and in contrast to monitoring electrodes, however, stimulation electrodes generally require a larger skin surface contact in order to provide sufficient transcutaneous electrical current to effect a desired physiologic response.
Many devices are designed for lower-energy level stimulation applications alone, such as TENS, EMS, NMS, FES, and interferential and iontophoresis therapy. At least some stimulation electrodes are touted as combination electrodes, which can also function as high-energy level defibrillation electrodes. U.S. Pat. No. 5,824,033 issued to Ferrari and was assigned to Ludlow Corporation. The patent discloses a disposable, multifunction (stimulating or defibrillating), x-ray transmissive electrode capable of conducting energy sufficient for defibrillation and which has improved current density distribution between the electrode and the skin of the patient. See column 2, lines 7-13, of the ""033 patent. Ferrari notes that monitoring electrodes are incapable of conducting and distributing the high levels of energy required in transcutaneous stimulation and defibrillation electrodes; thus, an important distinction exists between high-energy stimulating or defibrillating electrodes and lower-energy stimulating or monitoring electrodes. See column 1, lines 29-32.
The disclosed electrode 10 includes an electrically conductive metalxe2x80x94metal chloride (e.g., silverxe2x80x94silver chloride) coating 23 applied to one side of a sheet electrode member 21. See column 3, lines 31-41. Ferrari teaches that the sheet electrode as coated with the electrically conductive metalxe2x80x94metal chloride is not alone capable of transmitting and distributing the high levels of energy encountered in defibrillation over the entire surface of the electrode member. See column 4, line 66 to column 5, line 4. Thus, a current distributing mat 27 is required and is adhered to the opposite side of the sheet electrode member.
The electrode member is a thin, flexible sheet of electrically conductive polymer film having a thickness of two to four mils (0.05 to 0.10 mm). The metalxe2x80x94metal chloride ink is applied in a layer or layers, by silk screening, and is preferably less than ten microns in thickness. See column 4, lines 17-30. The ink may be up to 1 mil (0.0254 mm) thick. The silk screen technique of applying the ink coating facilitates the application of multiple layers having different shapes and edge configurations to achieve a tiered effect. See column 10, lines 10-23.
The outer perimeter of the metalxe2x80x94metal chloride coating is spaced inward from the perimeter of the electrode member and outward from the perimeter of the mat. The metalxe2x80x94metal chloride coating is preferable formed in two layers 23xe2x80x2, 23xe2x80x3, each a few microns in thickness. In addition, the layers are serrated or undulated at their outer perimeter. See column 6, lines 12-45.
The electrical conductors 35 are multi-strand metal wires in which the unsheathed end portions 35a are strands that are spread out and fanned as shown in FIGS. 1 and 3 of the patent. The fanned ends are bonded to the surface of the mat by pressing them against the mat and folding the mat over the ends. Specifically, the wires are metallized carbon fiber tows with a metal (e.g., nickel or copper) coating. See column 6, line 46 to column 7, line 40.
C. Defibrillation Electrodes
In a malady called xe2x80x9cfibrillation,xe2x80x9d the normal contractions of a muscle are replaced by rapid, irregular twitchings of muscular fibers (or fibrils). Fibrillation commonly occurs in the atria or ventricles of the heart muscle; the normal, rhythmical contractions of the heart are replaced by rapid, irregular twitchings of the muscular heart wall. A remedy for fibrillation is called xe2x80x9cdefibrillation,xe2x80x9d a procedure which applies an electric shock to arrest the fibrillation of the cardiac muscle (atrial or ventricular) and restore the normal heart rhythm.
Defibrillation electrodes must be capable of conducting the high-energy level required for defibrillation, up to 360 Joules or more. Defibrillation electrodes must also distribute the energy over a relatively large area of the epidermis of the patient to achieve adequate current density distribution within the atria or ventricles. These characteristics are sufficiently important that governmental regulatory agencies and medical industry groups have established standards for defibrillation electrodes. In particular, the American National Standards Institute (ANSI) standards for defibrillation electrodes have been published by the Association for the Advancement of Medical Instrumentation (AAMI). The ANSI standards for the size of defibrillation electrodes recommend, for example, that the minimum active area of individual, self-adhesive electrodes used for adult defibrillation and pacing shall be at least 50 cm2 and that the total area of the two electrodes shall be at least 150 cm2.
Many of the stimulating electrodes that have been disclosed do not comply with all of the defibrillation standards. The specification for defibrillation recovery characteristics, which describes certain time-related, electrical dissipation properties of the electrode following repeated electrical shocks of defibrillation currents, is especially difficult for many electrodes to meet. The use of non-compliant electrodes would invite the possibility of an inordinate, life-threatening delay following defibrillation. This restriction severely limits the usefulness of such electrodes in a critical care environment. Accordingly, many of these products bear a caution label that they are not to be used where defibrillation is a possibility.
U.S. Pat. No. 4,852,571 issued to Gadsby et al. addresses some of the shortcomings of these electrodes. The ""571 patent discloses a disposable electrode which passes the electrical defibrillation requirements as specified by AAMI. The ""571 design requires two separate layers of conductive inks, however, comprising a xe2x80x9cdiscontinuous layerxe2x80x9d of silver/silver chloride ink over a layer of carbon ink, which must be applied in two separate manufacturing steps. Avoidance of this dual-step ink-application requirement, which decreases process control, is desirable.
U.S. Pat. No. 5,352,315 was issued to Carrier et al. and was assigned to Ludlow Corporation. The ""315 patent is directed to a biomedical electrode, suitable for defibrillation, that uses a conductive ink 7, 8 to provide varying impedances and at the same time is inexpensively produced and disposable as well. The conductive ink layer or layers may be of the silver and silver chloride type and may be applied by screen printing. The disclosed embodiments provide for the ink blends and ink amounts (i.e., ink thickness and ink pattern) to be varied so that the thickness and pattern provide a particular impedance value suited for the intended placement of the electrode at a particular body site.
The variation in impedance is preferably achieved by varying the ink surface coverage of the electrode. See column 8, lines 47-49. More specifically, the ink surface coverage of the backing material to which the ink is applied is between about 7% and 90% and, preferably, between about 14% and 28%. See column 6, lines 16-23. The thickness of the ink layer is generally in the range of 0.1 to 0.8 mils (0.00254 to 0.0203 mm). See column 6, lines 46-49.
In summary, the known defibrillation electrodes suffer from several shortcomings. Many of the electrodes, especially those incorporating costly snap connectors or bilayered inks which require dual-step ink application, are undesirably expensive to manufacture. Other known defibrillation electrodes have long defibrillation recovery times, impairing their ability to reliably function promptly after transmission of a defibrillation pulse through the electrode. Still other electrodes fail to compensate for impedance variances. Problems have been encountered with prior art defibrillation electrodes, particularly after application of repeated high-level defibrillation or cardiac pacing pulses, with irritation and burning of the patient""s skin due to high current density around the perimeter of electrodes. It also remains a problem to improve the x-ray transparency of defibrillation electrodes.
To overcome the shortcomings of known defibrillation and pacing electrodes, a new disposable medical electrode intended for high-energy defibrillation and pacing with energy dispersion characteristics is provided. An object of the present invention is to provide a safe defibrillation electrode or set of electrodes which can compensate for impedance variances and can be economically manufactured, preferably in a continuous, automated process. A related object is to provide an improved electrode that features control of current density. Another object is to provide an electrode capable of conducting energy sufficient for defibrillation, and which has improved current density distribution between the electrode and the skin surface of the patient to efficiently deliver the energy without burning the patient""s skin.
Yet another object is to provide an electrode that has an extremely low profile: when applied to the patient, the electrode lies substantially flat along the plane of the skin. A related object of the invention is to eliminate the rigid snap-type connector of the prior art and to create a medical electrode that is flexible over its entire area and thus more comfortable. All portions of the electrode are yielding and may conform to the patient""s skin. The electrode offers no points of pressure when compressed against the skin by clothing or a mattress.
Another object of the invention is to create a medical electrode that may be left in place during radiographic procedures. Because the dense portions of the snap-type connector have been eliminated, the electrode of the present invention offers virtually no attenuation to x-rays and the small amount of attenuation that is produced is extremely uniform over the electrode surface. The electrode is substantially x-ray transparent.
To achieve these and other objects, and in view of its purposes, the present invention provides a disposable medical electrode that delivers high-energy defibrillation or pacing stimulation and has energy dispersion characteristics. The electrode includes an electrically conductive, carbon-filled polymer electrode member with a top face and a bottom face. A fanned wire contacts the top face of the electrode member for delivering energy to and transmitting energy from the electrode. An electrically conductive, skin-compatible hydrogel is disposed on at least a major portion of the bottom face of the electrode member. An electrically conductive metal/metal chloride ink coating underlies at least a major portion of the hydrogel on the bottom face of the electrode member. The ink coating has (a) a center with a first amount of ink, (b) an inner edge defining the terminus of the center and a step at which the ink content of the ink coating drops from the first amount of ink to a lesser second amount of ink, (c) an outer edge defining the terminus of the ink, and (d) a predetermined gradient disposed between the inner edge at which the ink coating has the second amount of ink, and the outer edge at which the ink is substantially absent. Finally, the electrode includes a removable release carrier sheet underlying and covering the hydrogel and the electrically conductive ink coating before use of the electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.